处理云函数中的嵌套承诺

Question

我是使用 async-await 编写打字稿的新手，但我检索了所有折扣，检查是否有效或过期，如果过期则获取所有折扣产品，然后在响应中打印其 ID，我无法处理嵌套承诺的问题和获取打折产品的 ID 以作为响应显示

exports.terminateDiscount = functions.https.onRequest(async (req, res) => {
    try {
        const discountSnapshots = await admin.firestore().collection("Discounts").get();
        const promises = [];
        const today = Date.now();
        discountSnapshots.forEach(async discountSnapshot => {
            const startDate = +discountSnapshot.data().startDate.toMillis();
            const endDate = discountSnapshot.data().endDate;
            if (today > startDate && endDate !== null && today > endDate.toMillis()) { // discount expired
                promises.push(discountSnapshot.id); // print discount id only
                const discountProducts=await admin.firestore().collection("Products").where("discountId", "==", discountSnapshot.id).get();
                discountProducts.forEach(product => {
                     promises.push(product.id); // products id's not added in promises :(
                });
            }
        });
        res.send(await Promise.all(promises));
    } catch (error) {
    }
});

Answer 1

async/await 与.forEach 一起使用非常困难。简单来说，.forEach 对数组的每一项都调用一个回调，并且它始终是一个sync 进程。这意味着，您永远不会等到const discountProducts = await admin.firestore()... 完成，您会在调用discountSnapshots.forEach... 后立即返回。

Promise.all 如果你给它一个 promises 数组就可以正常工作，在你的代码中，你将一个字符串数组（我认为discountSnapshot.id 是一个字符串）传递给Promise.all，在这种情况下它将返回“原始”输入 - 字符串数组。

我建议在这种情况下使用Array.map，.map 函数返回一个数组，我们让它返回一个承诺数组，然后等待所有承诺完成。

exports.terminateDiscount = functions.https.onRequest(async (req, res) => {
  try {
    const discountSnapshots = await admin.firestore().collection("Discounts").get();
    const today = Date.now();

    const ids = []; // to store discount id or products id or both ????

    const promises = discountSnapshots.docs.map(async (discountSnapshot) => {
      const startDate = +discountSnapshot.data().startDate.toMillis();
      const endDate = discountSnapshot.data().endDate;
      if (today > startDate && endDate !== null && today > endDate.toMillis()) { // discount expired
        ids.push(discountSnapshot.id); // print discount id only ???
        const discountProducts = await admin.firestore().collection("Products").where("discountId", "==", discountSnapshot.id).get();
        discountProducts.forEach(product => {
          ids.push(product.id); // products id's ??? "print discount id only"
        });
      }
    });
    await Promise.all(promises); // wait until we fill the ids array, the ids has been filled.

    res.send(ids);
  } catch (error) {
  }
});